In heavy utility vehicles, a diesel engine, which drives the rear axle via a cardan shaft, is generally installed above the front axle. Owing to this arrangement, the load on the rear axle depends to a great extent on the loading of the utility vehicle. For this reason, the brake force which is applied by the driver via the brake pedal at the rear axle is limited in conventional brake systems by a pneumatic valve with an axial load sensor (ALB, automatically load-dependent brake). This ensures that, in the case of an unladen vehicle, the entire brake pressure which is desired by the driver is not present at the rear axle, and therefore the tendency of the rear axle to lock is reduced. In addition, a greater degree of braking comfort is possible and the service life of the rear axle brake is extended.
In addition to the automatically load-dependent brake (ALB), contemporary utility vehicles have an anti-lock brake system (ABS). This has a rotation speed sensor (inductive sensor) on each wheel, and a pressure control valve which can modulate the pressure set by the driver by the brake pedal. The ABS is activated when there is a tendency of a wheel to lock.
What is referred to as an EBD (electronic brake force distribution) function is believed to be discussed, for example, in DE 10 2006 045 317 A1, said EBD function implementing the automatically load-dependent brake (ALB function) with the aid of ABS pressure control valves, and therefore eliminating the need for the load valve. Since, in contrast to the automatically load-dependent brake (ALB), the ABS does not have a load sensor, the loading state is determined during braking with the aid of the speed difference between the front axle and the rear axle (vVA-vHA). The lower the rear axle load, the more slip occurs at the rear axle with the same brake operation, i.e. the greater the speed difference vVA-vHA. The difference between the laden and unladen vehicle has a significantly smaller effect at the front axle because of the arrangement of the engine. The speed difference vVA-vHA during braking can therefore be used as a measure for the loading. In other words, during a braking process, EBD blocks the rear axle brakes, and under certain circumstances also an individual rear wheel, from the further build-up of pressure, depending on the differential slip between the front and the rear axle, by activating the ABS valves. However, if the front axle brakes are defective, that is to say the front axle brakes more weakly than normal or even not at all, the speed difference vVA-vHA can become very large. The EBD function would then set the ABS valves at the rear axle to maintain pressure and limit the brake pressure at the rear axle. However, in this case, the utility vehicle would be underbraked. Furthermore, in the case of a vehicle which is laden only to a very small degree or unladen, the problem arises that the brake pressure regulation is not very sensitive since in this case the differential slip between the front axle and the rear axle becomes relatively large and a prespecified differential slip threshold value is quickly exceeded.
Patent document DE 10 2011 118 130 B4 therefore presents a method for electronically regulating the brake force distribution depending on a differential slip or differential speed between at least one wheel of the front axle and at least one wheel of the rear axle of a vehicle, in which, when a differential slip threshold value or a differential speed threshold value is exceeded by the differential slip or the differential speed, the brake pressure at the rear axle is limited. In this case, the differential slip threshold value or the differential speed threshold value is determined depending on the braking request. In other words, a relatively small differential slip threshold value is set in the case of a relatively low driver braking request, and a relatively large differential slip threshold value is set in the case of a relatively large braking request. The regulating logic system is therefore configured such that, during normal braking with a low or medium braking request and in the case of a vehicle with low loading or an unladen vehicle, an intervention takes place before the ABS regulation starts at the rear axle. Heavy braking operations are identified by the detected driver braking request. A more sensitive transition to ABS regulation is ensured by corresponding adaptation of the differential slip threshold depending on the detected driver braking request. The braking request by the driver is detected by a pressure sensor in a pneumatic rear axle brake circuit or rear axle channel and/or by an electrical signal of an electric brake value transmitter in a digital foot brake module. However, one disadvantage of this is that an electrical pressure sensor of this kind is a relatively expensive component.
Regulation of the brake force distribution (EBD) in the outlined manner, in which the difference between the brake slip at the front axle and the brake slip at the rear axle serves as a control variable therefore has the disadvantage that, in the event of a failure of the brake circuit at the front axle, the pressure limiting operation at the rear wheels by the EBD regulating arrangement starts too early. This disadvantage is noticeable, in particular, in the case of front axle/rear axle brake circuit division because, in this case, both front wheel brakes are connected to the same pressure medium circuit and consequently a failure of the two front wheel brakes is accompanied by excessively low pressure at the two rear wheels. In order that the vehicle deceleration of 30% in the event of a failure in the brake circuit required in accordance with ECE guidelines can also be fulfilled by the rear axle brake circuit alone, a brake force distribution with brake force reduction in the rear axle brake circuit has to be suppressed in a case of this kind of a defective front axle brake circuit being identified.
According to DE 43 09 243 A1 of this generic type, the disadvantages outlined above are avoided with the aid of a pressure switch or pressure sensor which is installed in the front wheel brake circuit or circuits if it is ensured that the EBD regulation operation takes place only if the pressure switch or pressure sensor indicates the presence of brake pressure in the front wheel brakes. If the signal is not present, an EBD regulation operation is suppressed and as a result ensures that any limiting of the rear wheel brake pressure by the EBD regulating arrangement does not take place. Therefore, an EBD regulation operation is possible only when the pressure switch is open because it is only then ensured that pressure is building up in the front axle. However, this safety measure does not take into account the fact that, although a front axle brake pressure is formed in the front axle brake circuit, the front axle brake does not deliver any front axle brake force or delivers an only insufficient front axle brake force on account of mechanical failure of the front axle brake, for example on account of unfavorable friction conditions between the friction partners of the friction brake or owing to failure of the brake mechanism. In a case of this kind, an electronic brake pressure distribution arrangement, for example in accordance with the differential slip, would establish a relatively high differential slip between the wheels of the front axle and the wheels of the rear axle even though there is no tendency for the brakes of the wheels at the rear axle to lock, and then limit the brake pressure in the rear axle brake circuit on account of the differential slip threshold value being exceeded. This would lead to an undesired loss in brake force and therefore to a relatively long braking travel.
In contrast, the invention is based on the object of further developing a method and an apparatus of the kind mentioned at the outset in such a way that as high a braking power as possible is still achieved at low production costs in the event of failure of the front axle brake circuit.
According to the invention, this object is achieved by the method and apparatus described herein.